America acquired overseas territories as a result of the Spanish-American War; however, this raised a legal concern. What was the legal issue?

The United States received the Philippines and the islands of Guam and Puerto Rico. But the war was about freeing Cuba, not seizing the Philippines. So, Filipino rebels led by Emilio Aguinaldo waged a 3-year insurrection against their new american colonizer. While the Spanish-American war lasted ten weeks, the Philippine insurrection nearly lasted three years and resulted in 4000 American lives.